During power generation, water is generated in fuel cells through electrochemical reaction of hydrogen with oxygen, hereafter the water thus generated may be referred to as ‘generated water’. For example, in a vehicle equipped with fuel cells, the fuel cells may be expected to start in a subfreezing environment. In this case, the generated water remaining in the fuel cells may be frozen to interrupt the smooth supply of fuel. This may lead to failure of power generation.
In a fuel cell system including fuel cells, one proposed technique drives the fuel cells in a heat retention mode in response to the user's selection in order to prevent the generated water from being frozen and stops the operation of the fuel cells in the heat retention mode when the ambient temperature reaches a sufficiently high level (see, for example, Japanese Patent Laid-Open No. 2005-108832, No. 2005-317211, No. 2007-95655, and No. 2006-79864).
The generated water that is not frozen but is accumulated in the vicinity of electrolyte membranes, this phenomenon is generally called ‘flooding’, may still interrupt the smooth supply of fuel and lower the performance of the fuel cells. In such cases, water content control may be performed to regulate the water content in the electrolyte membranes included in the fuel cells during operation of the fuel cells. A scavenging process may also be performed to remove the generated water remaining in the fuel cells in a stop state of the fuel cells. The fuel cells may be driven in the heat retention mode described above. The operation of the fuel cells in the heat retention mode prevents the generated water from being liquefied and thereby eliminates the state of flooding. Any of such flooding elimination processes requires the additional power consumption and thereby worsens the fuel consumption rate. Under the condition of the high ambient temperature, there is a low probability of the occurrence of flooding. The flooding elimination process is thus often forced to stop at the high ambient temperature. The freeze of the generated water is the phenomenon resulting from the occurrence of flooding. The process of preventing the freeze is thus equivalent to the flooding elimination process.
Even under the condition of the high ambient temperature, the operation of the fuel cells in the heat retention mode or the scavenging process and the water content control may be required in certain circumstances. For example, in the event of a failure arising in the fuel cells, it is desirable to drive the fuel cell system in the heat retention mode with the view to identifying the cause of the failure. The operation of the fuel cells in the heat retention mode can exclude the flooding as a possible cause of the failure. When there is still a failure in the fuel cells driven in the heat retention mode, the cause of the failure can be identified as a reason other than the flooding, for example, the occurrence of some cracking.